Various disk-like optical discs that can optically be recorded and/or reproduced have been so far put into practical use as an optical recording and reproducing medium. A read-only optical disc in which embossed pits corresponding to data are formed on a disc substrate in advance, a magneto-optical disc capable of recording data thereon by utilizing a magneto-optical effect and a phase-change type optical disc capable of recording data by utilizing a phase change of a recording film are available as such optical discs.
There have hitherto been proposed read-only optical discs such as a CD (Compact disc), an LD (Laser Disc) and a DVD (Digital Versatile Disc)-ROM (Read Only Memory), a DVD+RW or DVD-RW (registered trademarks of optical discs) such as a rewritable type MD (Mini Disc) of a magneto-optical disc in which discrete information patterns or tracking guide grooves, i.e., grooves are formed in advance as preformatted patterns and various kinds of optical recording and reproducing mediums such as MO and MD Data2 (registered trademarks of magneto-optical discs: Sony Corporation) of magneto-optical discs.
Grooves are generally formed along recording tracks in order to carry out mainly tracking servo, and an opening end between the grooves is generally referred to as a “land”.
Then, the optical recording and reproducing medium having grooves formed thereon is controlled in tracking servo by a tracking error signal based on a push-pull signal obtained from light that had been reflected and diffracted on the grooves. The push-pull signal can be obtained in such a manner that outputs from a plurality of photo-detectors disposed symmetrically with respect to the center of the track are calculated by predetermined equations after light reflected and diffracted on the grooves had been detected by the photo-detectors.
These optical discs had increased a recording density so far by increasing a reproducing resolution of an optical pickup mounted on a reproducing apparatus, i.e., optical system on which light for reproducing an optical disc becomes incident. Then, the reproducing resolution of the optical pickup had been so far optically realized by shortening a wavelength λ of laser light mainly used to reproduce data or by increasing a numerical aperture NA of an objective lens for focusing laser light on the optical disc.
In various kinds of optical recording mediums such as CD, MD, DVD-ROM, DVD+RW, MD Data2, the wavelengths λ of reproducing laser light, the numerical apertures NA and the track pitches Tp are shown on the following table 1. A study of this table reveals that the wavelength λ of the reproducing laser light was shortened and the numerical aperture NA of the reproducing lens was increased so that the track pitch could be reduced to achieve a high recording density.
TABLE 1LaserNumericalwavelengthapertureTrackλ [nm]NAλ/2NApitch [nm]CD, MD7800.458671600DVD − ROM6500.60542740DVD + RW6500.60542800MD Data26500.52625950
A lower limit of the track pitch is selected in response to a cut-off frequency of the optical pickup of the reproducing apparatus and 2NA/λ. In actual practice, the lower limit of the track pitch is selected in a range of from approximately twice to 3/2 times the track pitch corresponding to 2NA/λ. The reason for this is to obtain a tracking reproducing signal with amplitude large enough to carry out stable tracking servo.
The high recording density optical disc such as DVD+RW and MD Data2 uses the push-pull signal as the tracking reproducing signal in recent years. Under existing circumstances, a push-pull signal amplitude value (Peak-to-Peak value) large enough to carry out stable tracking servo is higher than approximately 0.15.
As FIG. 8, which is a plan view, shows schematically an arrangement of the MD Data2, for example, double spiral grooves of ordinary groove (straight groove) 81 and a wobbling groove 82 are formed in such a manner that a track pitch Tp is 0.95 μm and that a track period Td serving as a period between the grooves is 1.90 μm. Land portions between the two grooves are formed as tracks 83 and 84 and thereby used as recording areas. The push-pull signal amplitude amount is approximately 0.30 to produce a sufficient signal amplitude amount so that the stable tracking servo characteristics can be realized.
In recent years, when a storage capacity of an optical disc further increases or a recording density of an optical disc further increases to miniaturize the optical disc, there has been proposed a so-called land-groove recording system in which recording areas are formed on both of the groove and the land. When the track pitch is 1.00 μm, the push-pull signal amplitude amount is approximately 0.48, and hence the stable tracking servo characteristic can be obtained.
However, when the recording areas are formed on both of the land and the groove, since only the side surface of the groove serves as an area in which data cannot be recorded, if the widths of the land and the groove themselves are reduced, then the side surface of this groove also is reduced in width and hence a cross-talk occurs in the adjacent recording areas. Then, a problem arises, in which the recording density of the optical disc cannot be increased more than a certain recording density.
Accordingly, the track pitch cannot be narrowed more than a certain degree substantially, i.e., a track pitch cannot be made close to a track pitch corresponding to a cut-off frequency that is determined by the wavelength λ of the reproducing laser light and the numerical aperture NA of the optical pickup.
In view of the aforesaid aspect, it is an object of the present invention to provide an optical recording and reproducing medium having an arrangement using a land-groove recording system in which a track pitch can be narrowed while suppressing a recorded signal from crossing the tracks of the land portion and the groove portion and which can stably produce a sufficiently large amplitude amount of a push-pull signal, an optical recording and reproducing medium manufacturing master disc for use in manufacturing this optical recording and reproducing medium and an optical recording and reproducing apparatus using this optical recording and reproducing medium.